Fix a harmless off-by-one error in counting controller argument lengths

Bugfix on 0.1.1.1-alpha; found by boboper.

changes/bytecount

@@ -0,0 +1,5 @@
+  o Minor bugfixes
+    - Fix a off-by-one error in calculating some controller command argument
+      lengths.  Fortunately, this is harmless, the controller code does
+      redundant NUL termination too.  Found by boboper.  Bugfix on
+      0.1.1.1-alpha.

src/or/control.c

@@ -2855,9 +2855,10 @@ connection_control_process_inbuf(control_connection_t *conn)
          && !TOR_ISSPACE(conn->incoming_cmd[cmd_len]))
     ++cmd_len;
 
-  data_len -= cmd_len;
   conn->incoming_cmd[cmd_len]='\0';
   args = conn->incoming_cmd+cmd_len+1;
+  tor_assert(data_len>(size_t)cmd_len);
+  data_len -= (cmd_len+1); /* skip the command and NUL we added after it */
   while (*args == ' ' || *args == '\t') {
     ++args;
     --data_len;